In military operations it is frequently desirable to dispense from a vehicle, such as a helicopter or a truck, sets of cargo items such as mines, to be distributed over the area being traversed by the vehicle.
A known way of doing this is to stow the items in a tube having a first, generally closed end, a sliding obturator being provided between the cargo and the closed end. When gas pressure is provided against the obturator, it moves in the tube and the cargo items are dispensed at the other end of the tube.
To provide the necessary pressure there has been developed what is known as a high-pressure/low-pressure system. Here a high pressure volume is in communication with a low-pressure volume, the latter comprising the space in the tube between the obturator and the closed end, and the former being a chamber outside the closed end of the tube and containing a shell. Discharge of the propulsion member creates a quantity of gas at high pressure in the first named volume, which passes to the second named volume and drives the obturator to expel the cargo elements. It has been convenient to trigger the discharge of the shell by an electric primer apposed to a percussion primer in the shell.
For reasons of safety during storage, transportation, and handling, means must be provided to prevent any premature operation of the electric primer from setting off the propulsion member. Such means must be capable of disabling the electric primer from triggering the percussive primer in a first condition, and of acting as a firing pin between the electric primer and the percussion primer in a second condition.